It seems like the first large blog to go down this morning belonged to The Daily Dot, which helped spread the infected posts to hundreds, if not thousands, of other blogs. The posts are signed by the GNAA, a notoriously inflammatory hacker group.
First, here's how to avoid getting hacked:

1) Maybe just don't go to Tumblr right now? This will be fixed soon, probably, but the exploit posts are embarrassing, and the more people get infected, the faster they spread.